Scientists at the University of Veterinary Medicine, Vienna, have found a correlation between hormone levels and personalities in birds.

For the study, the team looked at a group of birds that were characterized as "fast" or "slow" explorers. Fast birds are those that are "proactive," more aggressive and quicker to explore new surroundings, and "slow" birds are more cautious and "reactive." The birds used in the study had been bred selectively to create fast and slow lines of birds.

A Great tit nestling, such as those used in the study. (Photo by Michaela Hölzl, University of Veterinary Medicine, Vienna)

Researcher Mareike Stöwe investigated the relationship between stress hormone (glucocorticoid) levels and whether a bird was a fast or slow explorer. She found significant differences in the levels of the hormones in fast and slow birds.

According to the study, slow nestlings excreted more glucocorticoids than fast ones. Stöwe also found that subjecting nestlings to stress (handling) caused an increase in the amounts of glucocorticoids they excreted. The increase was more dramatic in fast nestlings than in slow ones, suggesting that more "proactive" birds responded more intensely to stress than less curious birds.

"Both nestlings and adults of the slow line produced higher baseline immunoreactive corticosterone metabolites (CM) values than fast-line birds," the study says. "Fast-line nestlings excreted lower baseline CM than nestlings of a wild population not selected for fast or slow exploration. Slow-line nestlings did not."